The generation of elastic waves in a target is a well-characterized phenomenon. It is known that when transient changes in the structure of a target occur, elastic waves are generated both on the surface and in the bulk of the target. Referring to FIGS. 1a-1d, there are four types of waves which can propagate in a target, such as a solid 1, namely longitudinal, shear, Rayleigh or surface, and Lamb waves (shown in FIG. 10a). The longitudinal and/or shear waves travel only through the bulk of the solid, with the longitudinal waves having a velocity that is approximately twice that of the shear waves. The Rayleigh waves travel only on the surface of the solid with velocities slightly less than that of the shear waves. The Lamb waves are supported by and propagate through very thin solids, and may be used to measure the thickness of the solid 1. Longitudinal bulk waves and shear bulk waves have been extensively used for the detection of flaws, measurements of elastic properties of solids, and for the monitoring of phase transitions, such as occurs when a molten metal solidifies. It is also well-known to measure the temperature of the solid 1, as the temperature effects the velocity of the waves within the solid.
A number of different types of transducers have been employed to generate elastic or ultrasound wave energy in solids. Of most interest herein is the use of a laser (e.g., impulse laser 2) to generate ultrasound waves, coupled with the use of a detection laser 3, such as is found in an optical interferometer, to detect a movement of the surface of the solid 1 in response to the propagating ultrasound waves.
For example, by synchronizing the operation of the interferometer 3 with the firing of the impulse laser 2, and by determining a difference between the impulse laser firing time and the time that the wave is detected, the velocity of the wave in the solid 1 can be determined; so long as the distance d is known between the spot where the impulse laser beam 2a impinges and where the detection laser beam 3a impinges. The determined velocity, or `time of flight`, may then be correlated with some property of interest of the solid, such as the structure of the solid or the temperature of the solid. For the case where the impulse laser beam 2a and the detection laser beam 3a are directed to opposite sides of the solid, as in FIG. 1d, it is possible to measure the thickness of the solid. The thickness can also be measured, with the impulse and detection laser beams impinging on the same side, if the solid is thin enough to support a Lamb wave.
A representative, but not exhaustive, list of U.S. Patents in this and related technical areas include the following: U.S. Pat. No. 3,601,490, issued Aug. 24, 1971 to K. Erickson and entitled "Laser Interferometer"; U.S. Pat. No. 3,694,088, issued Sep. 26, 1972 to J. Gallagher et al. and entitled "Wavefront Measurement"; and U.S. Pat. No. 4,633,715, issued Jan. 6, 1987 to J. Monchalin and entitled "Laser Heterodyne Interferometric Method and System for Measuring Ultrasonic Displacements".
Also of interest is U.S. Pat. No. 5,286,313, issued Feb. 15, 1994 to Thomas J. Schultz, Petros A. Kotidis (an inventor of the subject matter of this patent application), Jaime A. Woodroffe (an inventor of the subject matter of this patent application), and Peter S. Rostler. The subject matter of this U.S. Patent, entitled "Process Control System Using Polarizing Interferometer", is incorporated by reference herein. The preferred embodiment of the system described in this patent employs an XeCl impulse laser in combination with a Helium-Neon-based polarizing interferometer to provide, by example, remote detection of a temperature of a workpiece.
One intended operating environment for this type of system is in a metals fabrication and/or treating facility. As can be appreciated, and because of the ambient heat, vibration and airborne particulate matter that is typically found in this type of environment, severe demands and operating stresses are placed on the interferometer and its associated detection laser and optical elements.
Another important consideration is the cost of the system, as an industrial application may require the use of a number of materials analysis systems. That is, it is desirable to provide a rugged, compact and low cost system without compromising measurement accuracy and repeatability.
Although the system described in U.S. Pat. No. 5,286,313 is well-suited for use in its intended application, it is an object of this invention to provide an improved laser ultrasonics materials characterization and analysis system.